Conventionally, an electrophotographic image forming device such as an electrophotographic copying machine and a printer is provided with a fixing device. A generally known fixing device, as shown in FIG. 25, includes: a heating roller 501 which is heated by heating means; and a pressing roller 502 pressing the heating roller at a predetermined contact pressure, wherein a recording paper P, on which unfixed developer (for example, toner T) is transcribed by a transcription device provided at a preceding stage of the fixing device so that the unfixed developer is positioned on a surface in the side of the heating roller 501, is made to pass between the heating roller 501 and the pressing roller 502, thereby fixing the unfixed developer on the recording paper.
Further, an example of the electrophotographic image forming device is a color image forming device using developer of plural colors. However, in case of the electrophotographic image forming device, the developer of plural colors is transcribed on the recording paper, so that a thickness of an unfixed developer layer is large. Hence, in the aforementioned conventional fixing device, there occurs such phenomenon that a part of fused developer (or foreign substances such as paper dusts of the recording paper) adheres to the heating roller and the pressing roller.
In order to solve the foregoing problem, there is proposed a cleaning device in which: a roller-shape or scraper-shape (blade-shape) cleaning member is disposed so as to press the heating roller and the pressing roller, thereby removing unnecessary developer and foreign substances that adhere to the heating roller and the pressing roller.
Temperature of the heating roller of the fixing device is controlled so that its surface temperature is kept at set temperature in order to fix the unfixed developer. For example, in case where a softening point of the developer used ranges from 60 to 80° C. and a fusing point of the developer ranges from 130 to 180° C., the set temperature of the surface of the heating roller ranges from 180 to 210° C. In the fixing device using a cleaning roller as the cleaning member, the cleaning roller is directly in contact with the extremely hot surface of the heating roller, so that the surface temperature of the cleaning roller is substantially the same as the set temperature of the surface of the heating roller.
Thus, surface temperature of the cleaning roller exceeds the fusing point of the developer, and the unnecessary developer on the heating roller that has been collected by the cleaning roller is in a fusing state, so that the developer remains on the surface of the cleaning roller.
Therefore, after the cleaning roller collects a predetermined saturated amount of developer, it is impossible to cause the cleaning roller to collect the unnecessary developer from the heating roller, so that the excessive developer remains on the heating roller. This causes the heating roller and the pressing roller to be tainted.
Patent Document 1 (Japanese Publication for Unexamined Publication No. 162171/2003)(Tokukai 2003-162171)(published on Jun. 6, 2003) discloses the following arrangement in order to surely prevent front and rear surfaces of the recording paper from being tainted by surely collecting unnecessary developer from the heating roller and the pressing roller. That is, a fixing device cleaning device includes a pressing roller for pressing a heating roller at a predetermined contact pressure, wherein the cleaning roller (made of metal) is disposed so as to press the heating roller. Further, an axis-direction length of the cleaning roller is shorter than an axis-direction length of the heating roller, and is longer than a maximum width of a sheet.
According to the arrangement, developer which adheres from the recording paper to the heating roller moves from the heating roller to the pressing roller, and is collected by the cleaning roller, at the time of fixing the developer on the recording paper.
In this case, materials of the heating roller surface and the pressing roller surface are appropriately selected, so that it is possible to move substantially all the developer adhering to the heating roller to the pressing roller.
Further, temperature of the pressing roller is lower than temperature of the heating roller, and temperature of the cleaning roller is further lower than the temperature of the pressing roller. Thus, it is possible to make the temperature of the pressing roller lower than the fusing point of the developer.
Thus, the developer remaining on the pressing roller is collected by the cleaning roller with the developer in a semi-fusing state, and the developer is moved outward in the axis directions of both the rollers due to the contact pressure of the pressing roller and the cleaning roller, and the developer is discharged from both ends of the cleaning roller since the length of the cleaning roller is shorter than the length of the pressing roller.
Thus, the developer hardly remains on the surface of the cleaning roller, and substantially all the developer remaining on the surface of the pressing roller which corresponds to the length of the cleaning roller is collected by the cleaning roller.
Further, Patent Document 6 (Japanese Publication for Unexamined Publication No. 113736/1993)(Tokukaihei 5-113736)(published on May 7, 1993) discloses a cleaning roller provided with protruding portions in a spiral manner. When the cleaning roller is used, the protruding portions lead the developer to an axis end of the cleaning roller, thereby discharging the developer.
Further, Patent Document 7 (Japanese Publication for Unexamined Publication No. 66938/2001)(Tokukai 2001-66938)(published on Mar. 16, 2001) discloses a cleaning roller arranged so that: holes are formed on a surface of a metallic cylinder, and the holes are connected to a hollow provided in the metallic cylinder. When the cleaning roller is used, developer which has moved from the fixing roller or the pressing roller and adheres to the cleaning roller is stored into the holes of the cleaning roller, so that it is possible to prevent the developer deposited on the cleaning roller from flowing back to the pressing roller.
Further, each of Patent Document 2 (Japanese Publication for Unexamined Publication No. 104851/1985)(Jitsukaisho 60-104851)(published on Jul. 17, 1985), Patent Document 3 (Japanese Publication for Unexamined Publication No. 203473/1983)(Tokukaisho 58-203473)(published on Nov. 26, 1983), Patent Document 4 (Japanese Publication for Unexamined Publication No. 216368/1985)(Tokukaisho 60-216368)(published on Oct. 29, 1985), Patent Document 5 (Japanese Publication for Unexamined Publication No. 304080/2002)(Tokukai 2002-304080)(published on Oct. 18, 2002), and the like discloses a cleaning device which is arranged so that: a scraper-shape (blade-shape) cleaning member is in contact with the heating roller and the pressing roller, and toner and paper dusts that adhere to the heating roller and the pressing roller are scraped by the cleaning member. FIG. 25 shows a condition under which a scraper-shape (blade-shape) cleaning member 503 is pressed against a pressing roller 502.
Note that, in FIG. 25, the reference sign 504 indicates heating means constituted of a halogen heater or the like and the reference sign Y indicates a fixing nip. Further, FIG. 26 is a front view of the scraper 503, and the reference sign N indicates a position at which the scraper 503 and the pressing roller 502 are in contact with each other, and the reference sign Wp indicates a maximum width of the recording paper P.
However, according to the fixing device using the cleaning roller recited in Patent Documents 1 and 6, in case of using an image forming device which covers a paper having a large maximum width or in case of sequentially printing images (for example, in case of printing images on a large amount of sheets) by using a high speed machine in order to sequentially print images on wide recording papers such as A3-size recording papers for example, a pace at which the developer is discharged from both ends of the cleaning roller tends to be slow.
Thus, even in case of printing images right after turning ON the power source, the developer remaining on the cleaning roller returns from the cleaning roller to the pressing roller, so that the developer taints front and rear surfaces of a sheet on which an image has been formed. It was clarified that this phenomenon is conspicuous particularly in double-side printing based on image data whose printing ratio is high.
Further, the cleaning roller of Patent Document 7 which is formed in a cylindrical shape is in contact with another cylindrical roller, so that its scraping performance is lower than the case of using the scraper recited in Patent Documents 2 to 5.
Further, according to the fixing device using the scraper as shown in Patent Documents 2 to 5, it is necessary to surely bring the edge portion E of the scraper (blade) 503 into contact with a surface of the fixing member such as the heating roller 501 and the pressing roller 502, as shown in FIG. 27, in order to obtain a sufficient scraping performance. However, lot unevenness occurs due to size unevenness of parts, thermal deformation of parts, and the like, so that it is difficult to always stably bring the edge portion E of the scraper 503 into contact with the fixing member.
Particularly, in case where the scraper 503 and a holding member 505 for holding the scraper 503 are different from each other in terms of a coefficient of thermal expansion, the scraper 503 swings due to the thermal expansion, so that it is difficult to evenly bring the scraper 503 into contact with the fixing member also in a width direction.
Further, the edge portion E of the scraper 503 breaks into the fixing members so that the surface of the fixing member is damaged, and there is limit in making a thickness of the scraper 503 thinner. This results in such problem that: a high pressure at which the scraper 503 is pressed is likely to wear out the surface of the pressing roller 502.
Further, in the scraper-type cleaning device, the scraped toner and paper dusts remain an end of the scraper 503, so that a toner puddle DM1 is formed (FIG. 27). Further, when the toner puddle DM1 grows, the toner and the paper dusts move (flow) from the toner puddle DM1 to the surface of the fixing member, so that the recording paper P is tainted.
In order to prevent the toner puddle DM1 from flowing and tainting the fixing member, Patent Document 5 discloses a technique for preventing the toner puddle DM1 formed on an end of the blade from flowing to the fixing member by providing deformation following means which is brought into contact with a deformed portion of a pressed area (fixing nip portion Y) of the fixing member in a stoppage state so that the deformation following means follows the cleaning blade in a rotation state.
However, the toner adhering to the fixing member such as the heating roller, the pressing roller, and the like is fused by heat of the fixing member, so that it is difficult to completely scrape the toner D that has been fused and fixed on the surface of the fixing member even when the edge portion E of the scraper 503 is brought into contact with the fixing member as shown in FIG. 27. Thus, a part of the toner escapes from the edge E, and the toner adheres and is deposited on a scraper face S2 positioned in a vicinity of downstream with respect to the edge portion E. As a result, a second toner puddle DM2 is formed. Likewise, the paper dusts are mixed with the fused toner D, so that the paper dusts escape from the blade. Thus, the paper dusts are absorbed by the second toner puddle DM2.
As a result, the toner and the paper dusts flow from the second toner puddle DM2 to the surface of the fixing member, so that the recording paper P is tainted.
Particularly, in case where recording papers smaller than a width of the fixing member are subsequently made to pass, the toner puddle DM2 is fused and flows due to abnormal rise of temperature in an area which does not allow a paper to pass (non-paper passing area) in the fixing member. When a recording paper having a larger size is allowed to pass thereafter, the recording paper is conspicuously tainted.
Further, according to the cleaning device of Patent Document 4, a surface of a cleaning blade which is less likely to be released than the fixing member is brought into contact with the fixing member, so that it is possible to clear the toner without bringing an edge portion of the cleaning blade into contact with the fixing member. Moreover, the heating roller and the pressing roller are disposed so that they can be disconnected or connected, and displacement of the pressing roller which occurs in disconnection or connection of both the rollers causes a bending amount of the cleaning blade to change, thereby exfoliating and dropping the toner puddle DM2 adhering to the cleaning blade.
However, in case where the surface of the cleaning blade is brought into contact with the fixing member, the cleaning performance greatly drops compared with the case where the edge portion of the cleaning blade is brought into contact with the fixing member. In this case, it is necessary to provide the pressing roller so that the pressing roller can be disconnected and connected from and to the heating roller, so that the device is complicated.